Personal Homepages

Physics of computation. Design and realization of architectures for new forms of information processing, especially quantum computing. Theoretical work relating to the design, growth and characterization of solid state nanostructures for computation, with particular current emphasis on (a) quantum dots systems, both self assembled and lithographically defined, and (b) fullerene systems (nanotubes, endohedral C60, etc.) Secondary interest in other areas of quantum information theory, such as quantum game theory.

Spin amplification for quantum technologies
Dr. S.C. Benjamin, Dr. B.W. Lovett*, T. Close
This project involves analytic and numerical simulation of ordered spin arrays in order to predict the experimental parameters for spin amplification, i.e. having a single electron (or nuclear) spin state 'copied' (not in the cloning sense) onto a massive number of ancillas in a quick and robust way. This is one tactic to overcome the problem that electron spins, while acting as superb memory units for quantum information, cannot be directly measured as individuals. (*Heriot-Watt University)

Coherent Control of Spin Systems
Dr. B.W. Lovett*, Dr. S.C. Benjamin, Dr. E.M. Gauger
We are studying the quantum properties of nuclear and electron spins, primarily in molecular systems. Our aim is to provide theory that will allow for the control small numbers of spins, such that the quantum coherence is preserved for as long as possible. We collaborate with the Quantum Spin Dynamics experimental group (http://qsd.physics.ox.ac.uk/), and together we demonstrated that the quantum state of an electron spin can be transferred coherently to a nuclear spin, thus increasing the coherence time. We are now working on optical methods for further improving coherence, and for coupling several spins together. (*Heriot-Watt University)

Measurement Based Quantum Computing
Dr. B.W. Lovett*, Dr. S.C. Benjamin, Dr. J. Fitzsimons, Y. Matsuzaki
One can regard quantum entanglement as the fundamental resource needed in order to execute quantum algorithms. Certain kinds of entangled states exist which are universal resources, in the sense that any quantum algorithm can be performed simply by performing a prescribed series of quantum measurements. Moreover, even the entangled state itself can by created by making measurements. These insights have led to many new possible implementations of quantum computers, for example: one that uses only photons, one exploiting crossed atomic beams and others based on optical measurements on colour centres in diamond. Specific topics are: first principles physics of measurement, implementation of error correction or avoidance and entanglement creation by measurement. (*Heriot-Watt University)

Nanomaterials and quantum computing
Dr. B.W. Lovett*, Dr. S.C. Benjamin, Dr. E.M. Gauger
We are looking at how certain nanomaterials (such as quantum dots or crystal defects) can be used to implement quantum gate operations. We have developed methods for coherent quantum control of systems with a range of Hamiltonians. We are also interested in modelling decoherence, which is caused by the interaction of a system with its environment, and employs the theory of open quantum systems. We look at both Markovian and non-Markovian models of such open systems. We aim to provide experimental tests of the different theories working closely with the Low Dimensional Structures and Devices group at the University of Sheffield http://ldsd.group.shef.ac.uk/. (*Heriot-Watt University)

Colour centres in diamond as solid state single photon sources and quantum spin registers
F. Grazioso, P. Dolan, Dr. E. Abe, Dr. J.J.L. Morton, Dr. S.C. Benjamin, Dr. J.M. Smith
Diamond colour centres have demonstrated exquisite properties as single photon sources and quantum spin registers that operate even at room temperature, providing great opportunities for quantum communications and information technologies. This project involves using optical microscopy and spin resonance techniques to characterise the underlying physics and properties of single colour centres in new ultra-pure synthetic diamond material. Principal collaborations are with Element Six Ltd and the Diamond Trading Company.

Quantum superposition in large systems
Dr. S.C. Benjamin, E. Gauger, Professor G.A.D. Briggs, G. Knee
This is a theoretical project looking at the possibilities inherent in creating quantum superpositions of large objects such as massive molecules or SQUIDs and similar. A key theoretical tool will be the Leggett-Garg inequality.

6 public active projects

Erik M. Gauger, Elisabeth Rieper, John J. L. Morton, Simon C. Benjamin and Vlatko Vedral, Sustained Quantum Coherence and Entanglement in the Avian Compass', Phys. Rev. Lett. 106, 040503 (2011)

Li, Y., Barrett, S. D., Stace, T. M. and Benjamin, S. C. (2010), 'Fault tolerant quantum computation with non-deterministic gates' Phys. Rev. Lett. 105, 250502 (2010).

Schaffry, M., Gauger, E. M., Morton, J. J. L., Fitzsimons, J., Benjamin, S. C. and Lovett, B. W., 'Quantum metrology with molecular ensembles' Phys. Rev. A 82, 042114 (2010).

Matsuzaki, Y., Benjamin, S. C. and Fitzsimons, J. (2010), 'Distributed quantum computation with arbitrarily poor photon detection' Phys. Rev. A 82, 010302(R) (2010).

Matsuzaki, Y., Benjamin, S. C. and Fitzsimons, J. (2010), 'Probabilistic Growth of Large Entangled States with Low Error Accumulation', Phys. Rev. Lett. 104, 050501 (2010).

Schaffry, M., et al (2010), 'Entangling Remote Nuclear Spins Linked by a Chromophore' Phys. Rev. Lett. 104, 200501 (2010).

Benjamin, S. C., Lovett, B. W., and Smith, J. M., 'Prospects for measurement-based quantum computing with solid state spin', Laser & Photon. Rev., DOI 10.1002/lpor.200810051 (2009).

Jones, J. A., Karlen, S. D., Fitzsimons, J., Ardavan, A., Benjamin, S. C. et al. (2010), 'Magnetic field sensing beyond the standard quantum limit using 10-spin NOON states', Science 324, 116 (2009).

Lovett, B. W. and Benjamin, S. C. (2009), 'Comment on "Multipartite Entanglement Among Single Spins in Diamond" ', Science 323, 1169c (2009).

Kolli, A., Benjamin, S. C., Coello, J. G., Bose, S. and Lovett, B. W. (2009) 'Large spin entangled current from a passive device' New J. Phys. 11 013018 (2009).

Kolli, A., Lovett, B. W., Benjamin, S. C. and Stace, T. M. (2009) Benjamin and T. M. Stace, 'Measurement-based approach to entanglement generation in coupled quantum dots', Phys. Rev. B 79, 035315 (2009).

2008 and 2007 to be input.

Benjamin, S.C., Ardavan, A., Andrew, G., Briggs, D., Britz, D.A., Gunlycke, D., Jefferson, J., Jones, M.A.G., Leigh, D.F., Lovett, B.W., Khlobystov, A.N., Lyon, S.A., Morton, J.J.L., Porfyrakis, K., Sambrook, M.R. and Tyryshkin, A.M. (2006). 'Towards a fullerene-based quantum computer' Journal of Physics-Condensed Matter 18(21) S867-S883.

Benjamin, S.C., Browne, D.E., Fitzsimons, J. and Morton, J.J.L. (2006). 'Brokered graph-state quantum computation' New Journal of Physics 8.

Morton, J.J.L., Tyryshkin, A.M., Ardavan, A., Benjamin, S.C., Porfyrakis, K., Lyon, S.A. and Briggs, G.A.D. (2006). 'Bang-bang control of fullerene qubits using ultrafast phase gates' Nature Physics 2(1) 40-43.

Tyryshkin, A.M., Morton, J.J.L., Benjamin, S.C., Ardavan, A., Briggs, G.A.D., Ager, J.W. and Lyon, S.A. (2006). 'Coherence of spin qubits in silicon' Journal of Physics-Condensed Matter 18(21) S783-S794.

Yung, M.H., Benjamin, S.C. and Bose, S. (2006). 'Processor core model for quantum computing' Physical Review Letters 96(22).

Benjamin, S.C. (2005). 'Comment on "Efficient high-fidelity quantum computation using matter qubits and linear optics"' Physical Review A 72(5).

Benjamin, S.C., Eisert, J. and Stace, T.M.: 'Optical generation of matter qubit graph states' New Journal Of Physics 7 (2005)

Benjamin, S.C. and Bose, S.: 'Quantum computing in arrays coupled by "always-on" interactions' Physical Review A 70 (3) (2004)

Benjamin, S.C., Lovett, B.W. and Reina, J.H.: 'Optical quantum computation with perpetually coupled spins' Physical Review A 70 (6) (2004)

Benjamin, S.C.: 'Multi-qubit gates in arrays coupled by 'always-on' interactions.' New Journal Of Physics 6 (2004) art. no.-61.

Ardavan A., Austwick M., Benjamin S.C., Briggs G.A.D., Dennis T.J.S., Ferguson A., Hasko D.G., Kanai M., Khlobystov A.N., Lovett B.W., Morley G.W., Oliver R.A., Pettifor D.G., Porfyrakis K., Reina J.H., Rice J.H., Smith J.D., Taylor R.A., Williams D.A., Adelmann C., Mariette H. and Hamers R.J.: 'Nanoscale solid-state quantum computing' Philosophical Transactions of the Royal Society of London Series A - Mathematical Physical and Engineering Sciences 361, 1473-1485 (2003).

Benjamin S.C. and Bose S.: 'Quantum computing with an always-on Heisenberg interaction' Physical Review Letters 90, art. no.-247901 (2003).

Quantum Information Processing
B W Lovett / S C Benjamin

The Quantum and Nanotechnologies Group (www.qunat.org) anticipates that they will be able to offer one or more doctoral studentships in the area of quantum information processing. The group has broad interests, ranging from detailed modelling of semiconductor structures through more abstract ideas related to designs for quantum computer architectures and extending to fundamental questions about the nature of quantum information and measurement. At the time of writing the following are active projects:

i) Measurement based quantum computing. One can regard quantum entanglement as the fundamental resource needed in order to execute quantum algorithms. Certain kinds of entangled states exist which are universal resources, in the sense that _any_ quantum algorithm can be performed simply by performing a prescribed series of quantum measurements. Moreover, even the entangled state itself can by created by making measurements. These insights have led to many new possible implementations of quantum computers, for example: one that uses only photons, one exploiting crossed atomic beams and others based on optical measurements on colour centres in diamond.

Specific topics are: first principles physics of measurement, implementation of error correction or avoidance and entanglement creation by measurement.

ii) Nanomaterials and quantum computing. We are looking at how certain nanomaterials (such as quantum dots, molecules or crystal defects) can be used to implement quantum gate operations. We have developed methods for coherent quantum control of systems with a range of Hamiltonians. We are also interested in modelling decoherence, which is caused by the interaction of a system with its environment, and employs the theory of open quantum systems.

iii) Spin chains. One of the most important questions in quantum information processing is how we might transmit information from one computer to another. We have been looking at at this might be done using one (or higher) dimensional arrays of interacting spins (or similar quantum two level systems). An important theme is to achieve is much as possible with minimal external control --- in other words, to exploit the 'natural' dynamics of the spin system as completely as possible.

Another potential application of a spin chain is as a globally controlled quantum memory element. We are interested in developing the theory of molecular quantum memories, for both interacting and independent molecular systems.

There are several collaborators on these projects, including Dr Tom Stace (University of Queensland), Prof Sougato Bose (University College London), and Prof Leong Chuan Kwek (National University of Singapore). Currently no specific funding is in place; however, a number of funding routes exist and we would be happy to advise strong students about how to explore these.

Also see homepages: Simon Benjamin Brendon Lovett

Also see a full listing of New projects available within the Department of Materials.